Ajax Simplified

Ajax can become complex as far as implementation, but the concept is quite simple.

This is a simple tutorial on Ajax that I hope will ease the fears of
those of you who think Ajax can be intimidating. Despite the meaning
of the term (Asynchronous JavaScript and XML), Ajax really revolves around
a very simple principle. It lets you manipulate the contents of a Web
page without having to reload the page. Here are the key steps involved
that exploit the power of Ajax:

Capture an event (such as when a user changes an edit field or
presses a button).

The event triggers JavaScript code, which sends a query to the Web server.

The JavaScript code retrieves results from the server.

The JavaScript code uses the results to change the contents of the Web page.

JavaScript accesses the Document Object Model (DOM) to change the
contents of a Web page without reloading the Web page. For example,
suppose your Web page contains the following element:

Total: <input type="text" id="total" />

The id portion of the HTML tag creates an element called total in
the DOM, the contents of which you can change via JavaScript with the
following JavaScript code:

document.getElementById('total').value = <some value>;

Web designers have been using this capability for a long time. The
real power in Ajax comes from the ability to calculate the value for
the total at the server side rather than at the client. To keep it simple,
here's an example that doesn't really involve any server activity other
than returning a result. This example presents a simple form that lets
you type in a zip code. When you change the value of the zip-code field,
the JavaScript code executes a PHP script at the server side that returns
the shipping cost to that zip code. The JavaScript code then modifies
the totalshipping field to reflect the server response.

The example page shown in Listing 1 contains only the most basic
elements of an Ajax page—the primary functions being getHTTPObject,
handleHttpResponse and updateShipping. The onChange event is what
triggers the JavaScript function updateShipping. You could use onBlur
instead, which would call updateShipping when you simply leave the zip-code field and it loses focus.

The getHTTPObject function is what allows you to make a page request
via JavaScript, and the updateShipping function performs the page request.
The handleHttpResponse function receives the input from the page request
and extracts the information in order to modify an element in the page
(in this case, the totalshipping field). These are the three basic
functions you need to perform an Ajax operation.

This first example avoids XML entirely. The following line of code grabs
the result as plain text:

results = http.responseText;

If you try out this code, you'll find that when you type a zip code (or
virtually anything, because the code does no error checking) and then leave
the field, the JavaScript automatically retrieves the value $5.00
from the PHP application and places the value in the Total Shipping field.

Keep in mind that the above example takes as many shortcuts as
possible to keep it simple. There is no error checking or error
handling whatsoever. There aren't even any HTML tag names, only ids.
For example, it would be more common to create an input field that reads
<input type="text" name="totalshipping" id="totalshipping" />. You
probably wouldn't place the shipping cost in a field that a person
could edit (although your form could re-validate the shipping when the
person clicked “purchase” to correct any user changes). In addition,
the example doesn't actually calculate a shipping cost. The URL in the above
code points to a simple PHP script that returns the text value
“$5.00”
(Listing 2). A real application would take the zip code and
use it to calculate the shipping cost and return that value. In short,
the example cuts every possible corner to isolate how Ajax works rather
than how one should code an Ajax application.

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